From the viewpoint of low transmission loss, silica glass fibers have been used as optical fibers for long-distance transmission. In order to maintain their transmission and mechanical characteristics in such optical fibers, a primary coating made of a relatively soft resin and a secondary coating made of a relatively hard resin are applied to the surface of the glass fiber.
Conventionally used is a coated optical fiber in which, at the same time when a glass fiber is drawn from an optical fiber preform, a primary coating resin is applied thereto and cured thereon so as to form a primary coating, and then a secondary coating resin is applied to and cured on the outer periphery of the primary coating so as to form a secondary coating. On the other hand, a recent study from the viewpoint of improving productivity is directed to a method in which both primary and secondary coating resins are simultaneously applied to the outer periphery of the glass fiber and are subsequently cured.
As such a method, for example, proposed is a method in which a plasticizer transferable between the primary and secondary coatings is added such that its transferred amount from the secondary coating to the primary coating is at least equivalent to the transferred amount in the opposite direction, thereby restraining the glass fiber and the primary coating from peeling off from each other (Japanese Patent Application Laid-Open Gazette No. Sho. 62-129805). Also proposed is a method in which the curing temperature of the primary coating resin is set higher than that of the secondary coating resin so that the primary coating resin cures before the secondary coating resin begins to cure, thereby restraining the glass fiber and the primary coating from peeling off from each other (Japanese Patent Application Laid-Open Gazette No. Sho. 63-315542).
In the former method of these conventional methods, however, the plasticizer preferentially added to the secondary coating may bleed, whereby the adhesion force at the interface between the secondary coating and coloring layer in the coated optical fiber tends to decrease. As a result, in a tape-like coated optical fiber, it becomes difficult to remove the coatings simultaneously, thus remarkably deteriorating the workability. In the latter method, on the other hand, since the resin is cured at a high temperature, its termination reaction becomes prevalent, thus increasing uncrosslinked low molecule ingredients. When its crosslinking density does not rise, its modulus of elasticity and breaking extension may decrease, whereby the function as a coating for attaining stable long-term protection tends to deteriorate.
It is an object of the present invention to provide a coated optical fiber in which primary and secondary coatings are simultaneously formed around the outer periphery of a glass fiber and in which the occurrence of of peeling can sufficiently be prevented at the interface between the primary coating resin and the glass fiber without preferentially adding a plasticizer into the secondary coating or without making the curing temperature of the primary coating resin higher than that of the secondary coating resin; and a method of making the same.